Vasa praevia is a condition where the fetal vessels are within the fetal membranes (chorioamniotic membranes) and travel across the internal cervical os. The fetal membranes surround the amniotic cavity and developing fetus. The fetal vessels consist of the two umbilical arteries and single umbilical vein.
Vasa translates from Latin as vessel. Praevia translates from Latin as “going before”. Vasa praevia is where the vessels are placed over internal cervical os, before the fetus.
Pathophysiology
Under normal circumstances, the umbilical cord containing the fetal vessels (umbilical arteries and vein) inserts directly into the placenta. The fetal vessels are always protected, either by the umbilical cord or by the placenta. The umbilical cord contains Wharton’s jelly. Wharton’s jelly is a layer of soft connective tissue that surrounds the blood vessels in the umbilical cord, offering protection.
There are two instances when the fetal vessels can be exposed, outside the protection of the umbilical cord or placenta:
- Velamentous umbilical cord is where the umbilical cord inserts into the chorioamniotic membranes, and the fetal vessels travel unprotected through the membranes before joining the placenta.
- An accessory lobe of the placenta (also known as a succenturiate lobe) is connected by fetal vessels that travel through the chorioamniotic membranes between the placental lobes.
In vasa praevia, the fetal vessels are exposed, outside the protection of the umbilical cord or the placenta. The fetal vessels travel through the chorioamniotic membranes, and pass across the internal cervical os (the inner opening of the cervix). These exposed vessels are prone to bleeding, particularly when the membranes are ruptured during labour and at birth. This can lead to dramatic fetal blood loss and death.
There are two types of vasa praevia:
- Type I vasa praevia – the fetal vessels are exposed as a velamentous umbilical cord
- Type II vasa praevia – the fetal vessels are exposed as they travel to an accessory placental lobe
Risk Factors
- Low lying placenta
- IVF pregnancy
- Multiple pregnancy
Presentation
Vasa praevia may be diagnosed by ultrasound during pregnancy. This is the ideal scenario, as it allows a planned caesarean section to reduce the risk of haemorrhage. However, ultrasound is not reliable, and it is often not possible to diagnose antenatally.
It may present with antepartum haemorrhage, with bleeding during the second or third trimester of pregnancy.
It may be detected by vaginal examination during labour, when pulsating fetal vessels are seen in the membranes through the dilated cervix.
Finally, it may be detected during labour when fetal distress and dark-red bleeding occur following rupture of the membranes. This carries a very high fetal mortality, even with emergency caesarean section.
Management
For asymptomatic women with vasa praevia, the RCOG guidelines (2018) recommend:
- Corticosteroids, given from 32 weeks gestation to mature the fetal lungs
- Elective caesarean section, planned for 34 – 36 weeks gestation
Where antepartum haemorrhage occurs, emergency caesarean section is required to deliver the fetus before death occurs.
After stillbirth or unexplained fetal compromise during delivery, the placenta is examined for evidence of vasa praevia as a possible cause.
Last updated September 2020